Residents of Blue Island were warned Saturday not to use swimming pools and to curtail other outdoor activities until officials figure out just what was in a cloud of white silt emitted by an oil refinery.

The synthetic clay substance was released about 7 p.m. Friday and landed on homes, yards and swimming pools in various parts of the south suburban city of 22,000 people. Officials said it was nontoxic but could be an irritant if inhaled.

It came from the refinery owned by Premcor Inc. of St. Louis, formerly known as Clark Refining, in unincorporated Worth Township.

Mayor Donald Peloquin said refinery officials agreed Saturday to hire a specialist to test water in the park district's pool, which was closed Saturday because the sandy substance was floating on the surface. The surrounding park also was closed.

The city is trying to get the refinery to test residents' pools, too. "At least tell us what we have to do to clean them," Peloquin said. "I'm sure the residents will want Premcor to pay for it."

An employee at the refinery said no one was available Saturday to discuss the incident. The company spokesperson in St. Louis did not respond to telephone calls.

At least one nearby businessman said he didn't notice any chemical cloud Friday night.

"I didn't see anything unusual. I didn't smell anything. That's sort of strange," said Stanley Jones, manager of Kartway Go-Kart Track, down the street from the refinery. "The wind was blowing, though, so it might have blown it off."

Jones said he received a call from fire officials Friday night asking him to close, but police said his business could remain open.